The present invention concerns a safety cooling installation for the water reactor of nuclear power stations.
It is known that a nuclear power station should be equipped with safety or safeguarding circuits whose function is to maintain, with a high level of reliability, the power station--and, in particular, the reactor--in a safe and stable state in the event of an accident. In particular, power stations which employ water as the functional cooling fluid are equipped with circuits which make it possible, in the event the primary circuit is depressurized, to inject water under pressure in order to cool the fuel and prevent it from melting and, in the majority of cases, the stations are provided with circuits which make it possible to inject a fine spray of water into the object of this spraying being to reduce the pressure in the containment when the latter is subjected to a sudden increase in pressure subsequent to a rupture in the reactor's circuit where extremely high pressures exist, namely pressures of the order of 150 bars.
The water injected by these safeguarding circuits is furnished by one or more reservoirs generally located outside the containment enclosing the reactor circuit. In the case of a rupture of the pressure barrier of the reactor circuit, the said safety or safeguarding circuits enter into service automatically. The water injected by the safety circuits mixed with the reactor fluid flows to the lower part of the containment, called the sump. Because of the enormously high rates at which it is required to inject the water, the external reservoirs are rapidly emptied, and, because it is unacceptable to operate with an open circuit due to the large amount of radioactive contaminants in the water recovered in the sump, it is necessary to recycle this water in such a way as to keep the reactor fuel completely immersed and thus prevent it from melting.
Passage from the "injection" phase to the "recirculation" phase is generally effected automatically after a period which can vary from 20 minutes to several hours, depending on the size of the break. Safety regulations make it necessary to assume that the water located in the containment's sump is at saturation, that is, under conditions of bulk boiling at 120.degree.-130.degree. C. Such conditions impose very severe constraints on the selection and disposition of the pumps serving to extract the water from the containment, and also on the arrangement of the pipes connecting the sump to the said aspirating pumps. In effect, the water should not attain this state of saturation at any point in the said pipes, the consequence of such a saturation being the formation of vapor locks which would block the recirculation and hence produce cavitation and rapid destruction of the pumps.
According to present-day design, the pumps are installed in a very deep auxiliary building which is, for example, located at depths of 5 to 10 meters below the sump and in such a way as to obtain the necessary pressure for the aspiration, taking into account the load losses in the pipeline connecting the sump to the pumps. Such a design may pose difficult civil engineering problems, depending on the nature of the terrain and the seismic conditions of the site.
Another solution consists in the use of vertical pumps mounted in a cylindrical housing let directly into the structure of the auxiliary building, the length of this housing making it possible to obtain the pressure necessary for aspiration without the need for a similarly deep building.
It is evident that these designs, while being technically acceptable, are extremely costly and pose very large installation problems.
Use has also been made of special pumps which are mounted directly inside the containment's sump. These extraction pumps make it possible to provide the necessary aspiration pressure to the safeguarding pumps. These pumps, which are installed at the bottom of the sump, are actuated either by an immersed motor or by a "dry" motor located at a height sufficient for it not to be immersed. Such solutions not only require specially designed and qualified active components, that is, components requiring electrical energy in order to function and also requiring regular maintenance, but are, moreover, in contradiction with the present safety regulations which require a design which makes possible the maintenance and repair of the active equipment during the period following the accident. Such maintenance is impossible with the above arrangements, as the containment is absolutely inaccessible for a period which could be many years on account of the very high radioactivity level reigning in it.
Modifying power stations equipped with containment recirculation pump in such a way as to bring them to a state of reliability conforming to present-day regulations poses extremely complex problems and non-negligible risks, resulting in the need for digging an excavation which is large by comparison with the containment. Such a modification would also shut down production for several months, entailing, in addition, enormous financial losses.